there are multiple ways to do this. its all based on how you want the user to view and input info. i would assume your first sheet would allow users to add a list item.
Layout: List, compact or tile view
Feature: filter by signed in user (settings privacy: public with email)
other: Add and edit
Filter this by signed in user.
Once you allow add, click the plus and add components for questions. Ask for the fruit and veggie (choice component), date (special value), how much eaten(text entry for each) and user email (special value). Also add a column to calculate with formulas in the sheet or math in the data editor. Add column to calculate days. Each of these are a separate column on the sheet.
This way you have one sheet so far for all users.
a choices sheet for a list of fruit, veggies and any other multiple choice question you may have(separate column). you can add an A to Z sort in the choice component to alphabetize your fruits and veggies from the app, no matter what order they are in, in the sheet.
You can use another sheet to add lists by weeks or add a filter or visibility condition by date on the first sheet.
This is the simplest way I can think of. Hope it helps.